Hydrotalcite is a naturally occurring mineral having the formula: EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O
Hydrotalcite-like materials or anionic clay minerals have similar structures and have the general formula: EQU [M.sup.II.sub.1-x M.sup.III.sub.x ](OH).sub.2. .sup.x /.sub.y A.sup.y-. MH.sub.2 O
where M.sup.II and M.sup.III are divalent and trivalent cations, respectively, and A is an anion. These materials belong to the pyroaurite-sjogrenite class of minerals and their crystal structure has been described in the literature (Allmann, R., Acta Cryst. (1968), B24, 972). They have been widely described in the literature (Cavani et al., "Catalysis Today", 11, 173(1991) and references therein). The most common approach to synthesis of hydrotalcites is by coprecipitation of the two cations under conditions of supersaturation (U.S. Pat. Nos. 4,165,339, 3,879,523) and references therein. They are also synthesized by reacting activated magnesia with an aqueous solution of sodium aluminate, carbonate, and hydroxyl ions (U.S. Pat. No. 4,904,457). It is well known that hydrotalcites prepared by the above procedures have a hexagonal plate-like crystal habit (Reichle, W. T., Chemtech, 1986, 58). When crystallized at room temperature the crystallites have a diameter of approximately of about 0.01 to 0.1 microns and can be grown to about 1 to 5 microns by hydrothermal treatment. In all cases, the ratio defined by the diameter to the thickness of hexagonal crystals in such synthetic materials of the prior art are in the range of about 5 to about 20. Scanning and transmission electron microscope (TEM) pictures of hydrotalcite with the hexagonal plate-like crystal morphology are shown in FIGS. 1a and 1b, respectively.
The reaction of a basic magnesium compound having a needle like structure with a suitable aluminum compound under basic conditions results in the synthesis of hydrotalcites with a needle like morphology (U.S. Pat. No. 4,351,814).
The term "hydrotalcite-like" is recognized in the art. It is defined and used in a manner consistent with usage herein in the comprehensive literature survey of the above-referenced Cavani et al article.